This invention relates in general to a cassette for use in computed radiography and more particularly to a computed radiography cassette for use in radiographic applications, such as mammography, where it is desirable to detect x-rays after their passage through the cassette for automatic x-ray exposure control and to obtain an x-ray image as close as possible to the chest wall.
In computed radiography, a storage phosphor plate has a radiographic image formed thereon by exposing an object, such as a body part, to x-rays. The exposed storage phosphor plate is then provided to a reader where the plate is stimulated with radiation of one light frequency to emit a radiation image of another light frequency. The emitted image is captured, converted to a digital radiographic image, and stored, displayed or otherwise used.
In order to minimize damage to the storage phosphor plate during use and to prevent undesirable ambient light exposure of the plate, the storage phosphor plate is normally contained in a sturdy light tight cassette. U.S. Pat. No. 5,276,333, issued Jan. 4, 1994, inventor Robertson, discloses a cassette including a shell having upper and lower panels, first and second side members, and a front member forming a five sided cavity having an open end. A storage phosphor assembly is removably mounted in the shell and includes a back member and a rigid insert plate cantilevered from the back member. A storage phosphor is disposed on the upper surface of the insert plate. The upper panel of the shell is made of x-ray transmissive material, such as a carbon fiber composite. The lower panel is of a more x-ray opaque material such as aluminum for rigidity. The insert plate of the storage phosphor assembly is also of more x-ray opaque, material such as an aluminum honeycomb panel having an aluminum honeycomb core sandwiched between aluminum skins. The side, back and front members are of extruded aluminum and thus also more opaque to x-ray transmission.
Although the latter cassette has been commercially successful and appropriate for its applications, a need has arisen to provide a computed radiography cassette that is more x-ray transmissive in applications, such as mammography, where x-rays passing through the cassette are detected for automatic exposure control (AEC) devices which control patient exposure to x-rays. The x-ray opaque material used in the insert panel and lower panel of the described cassette inhibit transmission through the cassette of low dose x-rays. Moreover, the width of the aluminum extrusion used in the front member, along with associated clearances between the patient chest wall and internal storage phosphor screen, inhibits the use of a standard cassette in mammography where the cassette front end is positioned against the chest wall during x-ray exposure.
According to the present invention, there is provided fulfillment to the needs discussed above.
According to a feature of the present invention, there is provided a computed radiography cassette comprising:
a shell including upper and lower panels joined to first and second side members and a front end member to form a five sided cavity having an open end;
a storage phosphor assembly including a back end member, an insert plate having an upper face and being cantilevered from said back end member, and an x-ray storage phosphor disposed on said upper face of said insert plate, wherein said storage phosphor assembly is removably contained in said shell such that said back end member closes off said open end of said shell;
wherein said upper panel is of x-ray transmissive material; and
wherein said insert plate and said lower panel are made of x-ray opaque material having aligned sections thereof of x-ray transmissive material which allow x-rays to pass through said sections to an x-ray detector adapted to be located adjacent to said cassette.
The invention has the following advantages.
1. A computed radiography cassette is provided that can be used in applications, such as mammography, using automatic x-ray exposure control to reduce patient exposure to x-rays.
2. A computed radiography cassette is provided that can be used to capture x-ray images close to the chest wall in mammography applications.
3. A modified computed radiography cassette is provided that can be used in existing CR readers with little or no modification to the readers.